The formation of silver halide grains comprises two main steps, the nucleation step and the crystal growth step. Under some conditions of precipitation two additional processes, Ostwald ripening and recrystallization, can occur.
As disclosed in T.H. James "The Theory of the Photographic Process", 4th Edition, Macmillan Publishing Co., Inc., New York, the nucleation step is the process in which there is a population explosion of the number of crystals when entirely new crystals are created. The crystal growth step represents the addition of new layers of silver halide to crystals that are already present. Ostwald ripening occurs principally at higher temperatures, in the presence of silver halide solvents, and where there is a wide distribution of grain sizes. Under these conditions a strong tendency exists for the smaller crystals to dissolve and the large crystals to grow even larger. Recrystallization is the process in which the composition of the crystals changes. This occurs when crystals of different composition are present, because the formation of a solid solution of intermediate composition is thermodynamically favored. The recrystallization process proceeds more rapidly when silver halide solvents are present because some of the ions must dissolve and diffuse through the emulsion to form the mixed crystal phase of intermediate composition.
The silver halide grains may be precipitated by a variety of conventional techniques. The most common techniques, widely employed in the photographic industry, are the single-jet and double-jet method.
In the single jet method a silver nitrate solution is added to a reaction vessel containing a solution of halides in a protective colloid, usually gelatin. In this method, because the varying concentration of halides in the solution determines which silver halide grains are formed, the formed silver halide grains are a mixture of different kinds of shapes and sizes. This process usually creates crystals with a certain number of lattice defects, useful to improve speed, but with disuniform or wide distribution of the particle sizes.
In the double-jet method a silver salt solution in water and a halide salt solution in water are concurrently added into a reaction vessel containing the dispersing medium, usually gelatin. Double-jet precipitation processes are described, for example, in GB 1,027,146, GB 1,302,405, U.S. Pat. Nos. 3,801,326, 4,046,376, 3,790,386, 3,897,935, 4,147,551, and 4,171,224. In the double jet method the size of the formed silver halide grains can be controlled by the integration of the reaction temperature, the silver and hydrogen concentration (pAg and pH), the relative mixing uniformity of the reactants and their concentration, the precipitation flow rate, the kind and concentration of the solvent existing in the gelatin solution.
Accordingly, the preparation of a silver halide emulsion of monodispersed particles having a narrow size distribution by balanced double-jet method must be performed keeping in mind all the above mentioned variables which affect the particle sizes.
References disclosing the influence of such parameters on the average particles size and size distribution of silver halide emulsions can be found in:
(1) I.H. Leubner "Formation of Silver Halide Crystals in Double-jet Precipitation:AgCl", Journal of Imaging Science, Vol 29, June 1985, PA1 (2) R.W. Strong and J.S. Wey, "The Growth of AgCl Crystals in Gelatin Solution", Photographic Science and Engineering", Vol. 23, June 1979, PA1 (3) D.F. Shiao, "Kinetic Modelling of Growing Silver Halide Microcrystals in Gelatin Solution", "Photographic Science and Engineering", Vol. 24, May 1980, PA1 (4) I.H. Leubner "A Novel Precipitation Technique to Control the Crystal Size Distribution of Silver Halide Emulsion", Journal of Imaging Science, Vol. 34, May 1990, PA1 (5) I.H. Leubner "Nucleation in Presence of Ostwald Ripening Agents", Journal of Imaging Science, Vol. 31, April 1989, PA1 (6) I.H. Leubner "Nucleation in Presence of Growth Restrainers", Journal of Crystal Growth, Vol. 84, 1989, pp. 496-502, PA1 (7) I.H. Leubner, "Crystal Formation under Kinetically Controlled and Diffusion Controlled Growth Conditions", Journal of Physical Chemistry, Vol. 91, 1987. PA1 (a) forming silver halide nuclei with single jet precipitation by adding a water-soluble silver salt solution to a reaction vessel containing a water-soluble halide salt solution in a hydrophilic colloid at a pCl of from 1.0 to 2.0 and at a temperature lower than 80.degree. C., the water-soluble silver salt solution being added in an amount of from 0.1 to 15% by weight of silver relative to the total silver, PA1 (b) stabilizing said silver halide nuclei to seed crystals for at least six minutes at a temperature lower than 80.degree. C. and at pCl of from 1.5 to 3.0 by Ostwald ripening, and PA1 (c) growing said seed crystals by double jet precipitation of silver and halide salt solutions at a constant chloride ions excess of from 30 to 70% mol and a temperature lower than 80.degree. C.
According to (1) the nucleation of silver chloride crystals in double jet precipitation is function of the reactant addition rate and solubility, pAg and temperature, while the number of stable nuclei increases with increasing the addition rate and decreases with increasing solubility and temperature. References (2), (3), and (4) describe the parameters influencing the growth of silver halide grains by double jet precipitation, such as, for example, nuclei size, temperature, pCl, and redissolution of preformed grains. The effect of Ostwald ripening, promoted by silver halide solvents, and of crystal growth restrainer is also disclosed in references (5) and (6).
A process for preparing silver halide emulsions with a narrow grain size distribution is described in EP 174,018, in which the addition of monodisperse seed crystals, made by balanced double jet addition, to a gelatin alkali metal halide solution is made prior the addition of the silver nitrate solution by single jet addition. The grain size is uniform and predictable, controlled by the size, number and distribution of the seed crystals and by the total amount of silver added during the process.
U.S. Pat. No. 4,539,290 discloses a process and an apparatus for double jet precipitation in which solutions of a concentrated silver salt and halide salt are mixed in a primary mixing zone defined within the reaction vessel. The process is characterized in that the silver and halide salt solutions are alternatively introduced, with a pulse of a predetermined volume, at substantially the same point in the primary mixing zone with a predetermined pause between each pulse.
U.S. Pat. No. 4,879,208 discloses a method to prepare silver halide emulsions having a uniform silver halide composition and size distribution which comprises providing a mixer outside a reaction vessel which contains an aqueous solution of protective colloid, and in which silver halide grains are grown, feeding an aqueous solution of silver nitrate and an aqueous solution of a water soluble halide into the mixer, stirring the two solutions in the mixer to form fine grains of silver halide, and immediately introducing the fine grains into the reaction vessel. WO 90/1462 discloses a similar method wherein nucleation of silver halide grains is caused in the reaction vessel. Further improvements and modifications of the above described methods are disclosed in U.S. Pat. No. 5,104,785, EP 374,853 and U.S. Pat. No. 5,035,991. In U.S. Pat. No. 5,104,785 both the nucleation and growth of silver halide grains are performed in the reaction vessel and the formation of the fine silver halide grains is controlled by the flow rate of the solutions being supplied to the mixer and the rotational speed of the mixer. In EP 374,853 the temperature of the protective colloid solution in the mixer is kept below 40.degree. C. and the protective colloid is gelatin having a molecular weight lower than 40,000. In U.S. Pat. No. 5,035,991 the control process further includes measuring the silver ion potential in the mixer or in the reaction vessel. In U.S. Pat. No. 5,004,679 the addition of protective colloid is performed together with the water-soluble silver or halide solution or independently.
U.S. Pat. No. 5,104,786 discloses a method for achieving uniform nucleation conditions such that the formation of younger nuclei is not influenced by the presence of older crystals. The nucleation is performed without backmixing with previously nucleated grains with a plug-flow process. The plug-flow process is carried out until to nucleation is completed and then the nucleated grains are transferred to a mixing container where they are ripened and grown. This method represents an improvement of the method described in the above mentioned U.S. Pat. No. 4,879,208, wherein a backflow in the mixer during nucleation takes place.
U.S. Pat. No. 4,339,532 describes a process to prepare a monodisperse negative working photosensitive silver halide emulsion comprising grains having a uniform habit and relatively high degree of crystal disorder by precipitating silver halide in the presence of a seed emulsion which is predominantly composed of silver chloride under conditions whereby substantially none of the silver chloride of said seed emulsion is redissolved and substantially no additional grains are formed, i.e., at a pAg of from 7 to 9.
In U.S. Pat. No. 4,242,445 a method to obtain a silver halide emulsion having a narrow grain size distribution is described. It consists in completing the nucleation of the silver halide crystals at the initial stage of grain formation and by increasing the concentrations of the aqueous solutions of the inorganic salts to such an extent that fresh nuclei are hardly produced during the period of grain growth.
EP 165,576 discloses a process for producing a monodispersed silver halide emulsion wherein in a first step polydisperse silver halide nuclei of silver halide comprising from 0 to 5 mol % of iodide are formed at pBr from -0.7 to 2.0, in a second step monodisperse silver halide seeds are formed by ripening, in the presence of a silver halide solvent, said silver halide nuclei, and in a third step the silver halide seeds are grown by addition of silver and halide salt solutions.
U.S. Pat. No. 4,269,927 discloses a high chloride silver halide emulsion comprising as a dopant at least one metal selected in the group of cadmium, lead, and zinc. Double-jet method is specifically preferred to single-jet method to obtain monodispersed silver halide emulsions.
As mentioned above, the preparation of monodispersed silver halide emulsion having a predetermined average grain size and grain size distribution by double jet method requires the control of a number of parameters. This requires expensive and complex control apparatus.
An aspect of the present invention is to provide a method for obtaining a monodispersed silver halide emulsion in which the average grain size and the grain size distribution are mainly controlled by the number of seed crystals precipitated and stabilized at an early stage of the process.